It has been confirmed that dioxins which are extremely poisonous are generated and discharged from the incineration apparatus for municipal waste, industrial waste and the like. Conventionally, in order to suppress the generation of dioxins, the amount of carbon monoxide (CO) generated is measured, and the combustion is controlled such that the measured amount of CO is reduced. Dioxins are a kind of hydrocarbon, and they are believed to be generated from an incompletely combusted matter and chlorine in the incineration step. CO is an index of the combustibility, that is to say, the generation of incompletely combusted matters.
Jpn. Pat. Appln. KOKAI Publication No. 5-99411 (to be referred to as "prior art document 1") discloses an example of the combustion control technique using the CO generating amount as an index. The prior art document 1 shows that the generation of incompletely combusted matters such as dioxins can be suppressed more effectively by controlling the combustion so as to reduce the amount of CO generated. A waste incineration apparatus, to which the technique disclosed in the prior art document 1 is applied, includes a control amount arithmetic unit and supply control means. The control amount arithmetic unit judges the excessive or insufficient amount of water sprayed into the combustion furnace, and of primary air supplied to the combustion furnace, from the temperature of the furnace and the amount of CO generated. Then, on the basis of these judgments, supply control signals for above amounts are generated. The supply control means serves to adjust the amount of water sprayed and the amount of primary air, in accordance with the supply control signals therefor.
Jpn. Pat. Appln. KOKAI Publication No. 4-288405 (to be referred to as "prior art document 2" discloses another example of the controlling method carried out with reference to the amount of CO generated as an index. In the prior art document 2, the exhaust gas from the waste combustion furnace is made to pass through a bag filter, and the amount of CO generated in the exhaust gas is measured. Thus, in this method, the temperature of the inlet of the bag filter is controlled in response to the measured amount of CO generation, so as to decrease the amount of dioxins in the exhaust gas.
Jpn. Pat. Appln. KOKAI Publication No. 5-312796 discloses a semi-continuous monitoring device for measuring the concentration of chlorinated aromatic compounds in exhaust gas, which correlate well with dioxins. In this device, the exhaust gas is subjected to a pre-treatment so as to remove coexistent moisture and dusts from the gas, and then is made to pass through an adsorption tube. Thus, chlorinated aromatic compounds, such as chlorobenzenes, contained in the exhaust gas are adsorbed on the adsorption tube to be concentrated. Then, the chlorobenzenes are detected with gas chromatography.
However, application of the amount of CO generated to an index for the control of combustion, as in the waste incineration apparatus disclosed in prior art document 1, may not always be accepted in all cases but be accepted in only limited cases.
In more detail, application of the CO generation amount to an index for the control of combustion is principally unreasonable for the following reasons.
That is, incompletely combusted matter generated when a combustible such as waste is burned, can be mainly divided into fatty compounds, aromatic compounds and chlorinated materials of those compounds. Generally or theoretically, for example, the bond dissociation energy for a carbon-carbon bond is lower in the aliphatic compounds than in the aromatic compounds. This is due to the resonance stabilization property of the aromatic compounds. Thus, the aliphatic compounds have more easily dissociative bonds, and therefore are more easily combustible.
If the combustibility is low because of a low temperature in the furnace due to the variation in the quality of waste or the like under a constant amount of primary air, incomplete combustion occurs and the CO concentration increases. In this case, it is estimated that both the aliphatic compounds and the aromatic compounds are combusted, and that the concentration of the incompletely combusted matter is high.
Further, if the combustibility is high because of a high temperature in the furnace under a constant amount of primary air, the shortage of the primary air occurs, and the CO concentration increases. In this case, the aliphatic compounds, which are more easily combustible, burns with a priority over the aromatic compounds. The aromatic compounds therefore remain unburned in relatively high amount.
Thus, the reason the CO concentration starts to increase slightly from a minimum point, at the high temperature of the furnace, is that the shortage of the primary air occurs due to the combustion of the aliphatic compounds with priority. It is expected that the increase in the CO concentration is not mainly due to the decomposition and combustion of the aromatic compounds which can generate dioxins. Thus, the increase in the CO concentration may indicate the shortage in the primary air, but may not always be an index of the generation or increase of incompletely combusted matter of the aromatic compounds or the like.
Further, in the waste incineration apparatus disclosed in prior art document 2, changes in the concentrations of dioxins are significantly influenced by the operating temperature of the bag filter.
The lower the operating temperature of the bag filter is, the smaller the amount of the exhausted dioxin is. However, combustion exhaust gas generated from a waste incineration apparatus contains harmful components such as SO.sub.X and HCl other than dioxins. If the bag filter is operated at a low temperature of about 160 to 200.degree. C. so as to collect dioxins by the bag filter, there raises a high possibility that the facilities such as the bag filter and pipes are corroded by harmful components such as SO.sub.X and HCl.
Further, when the bag filter is operated at a low exhaust gas temperature, for example, moisture in the exhaust gas condenses into water, and sulfuric acid and HCl generated by the chemical reaction of part of SO.sub.X dissolves into the water. Thus, when the bag filter is operated at a low temperature, the facilities such as the bag filter and the pipes may be corroded. In order to avoid this, when the concentration of dioxins in the exhaust gas which are generated by the combustion of waste in the incineration furnace is low, it is necessary to operate the bag filter such that the exhaust gas temperature at the inlet of the bag filter becomes as close as possible to a temperature of 200.degree. C. which is of a conventional case.
Further, the techniques disclosed in the prior art documents 1 and 2 entails the following drawback.
In the case of only the CO concentration being monitored as an index, the measurement of the CO concentration is easy. However, the CO concentration value does not contain any information regarding the chlorination reaction of aromatic compounds. Therefore, any information which directly reflects the chlorinated aromatic compounds such as dioxins cannot be obtained. The control of the combustion in such a way as to reduce the amount of CO generated can decrease the amount of incompletely combusted matter on the whole. In other words, the control of combustion with reference to the CO concentration as an index is effective if the level of the amount of incompletely combusted matter generated is high, as in the case of waste incineration apparatus manufactured several years ago. However, the control of combustion with reference to the CO concentration as an index cannot further suppress or reduce the amount of incompletely combusted matter generated, especially chlorinated aromatic compounds such as dioxins, if the level of the amount of incompletely combusted matter generated is extremely low (for example, CO concentration is equal to or less than 50 ppm), as in the case of the waste incineration apparatus of the latest type.
The present invention has been proposed in consideration of the above-described drawbacks of the conventional techniques, and the object of the invention is to provide an incineration apparatus capable of achieving the further suppression and reduction of dioxins, which cannot be achieved by the control of combustion with reference to the CO concentration as an index.